Introduction

The wind affects Long Island Sound (LIS) in several different, but very important, ways. The speed and direction of winds over the southern New England Shelf determine the magnitude of the non-tidal sea level fluctuations at the eastern end of LIS. Winds from the northeast (to the southwest) are most effective in generating coastal set-up, a positive sea level anomaly that rapidly propagates throughout LIS. Winds from this direction also cause an additional local set-up in the Sound that raises the water levels in the west relative to those in the east by up to several meters. This is a well-established effect and the mechanisms are comprehensively summarized in O’Donnell et al. (2014). This particular sensitivity to winds from the northeast is a consequence of the coastal geometry. It is responsible for the dramatic flooding that occurs in the western end of LIS during major storms. However, the effects of more modest storms are amplified in the same way and this affects the frequency of flooding of salt marshes.

In the western Sound the surface stratification is modulated by the vector component of the wind to the southwest. This is also now well established by both model results (Wilson et al., 2008) and direct observations (O’Donnell et al., 2008). Basically, the rate of restratification of the western sound can be modulated by wind. Winds from the southwest accelerate the rate, and winds from the northeast slow it. Recently, Wilson et al. (2015) has provided convincing evidence that the frequency of occurrence of wind from the northeast can influence the duration and extent of bottom water hypoxia.

It is clear that the ecosystems of LIS are sensitive to the statistics of the wind field, both in the summer when water column stratification is impacted, and in the winter when saltmarshes are frequently subjected to inundation. There are likely to be other effects as well. It is, therefore, important to characterize the statistics of long term trends and fluctuations. O’Donnell (2010) has already demonstrated that all the meteorological stations reporting more than 50 years observations in the New England states show a decline in the monthly averaged wind speed through 2005. .

Discussion

We have expanded the analysis of wind observations described by O’Donnell (2010) in which he found that there had been a regional reduction in the average wind speeds during the time of year when they are highest, February-April, at stations all across New England through 2005. We show by extending the analysis using the Bridgeport–Sikorski Airport data record that the decreasing trend has continued through 2015. Further, the average wind speeds during the calmer months, July-September, also shows a substantial decrease.

By examining the distributions of the estimates of the vector component of the wind stress, we show that they are also largest in February-April and smallest in July-September. Binning the vector components during these two intervals for each year, we computed the boundaries of the 95, 90 and 50 percentile intervals, and displayed in the figure to the right, their evolution since 1947. A dramatic decline in the 95 and 80% stress level was detected. In February to April of the 1960-80 interval, the magnitude of the stress during events that were less frequent than 5% was approximately 0.2 Pa. Since 2000 it has been 0.1 Pa. Summer statistics show a similar change.

(a) shows the boundaries of the stress intervals containing 95% (blue), 80% (green) and 50% (red) of the along LIS component stress estimates in February-April each year. (b) shows the same statistics but for the across LIS stress components. The data distribution evolution for the low wind July-September period are shown in (c) and (d) for the along and across Sound components respectively.

We should be concerned that trends at a single station might be the consequence of some local effect. Small movement of sensors or new buildings can change airflow patterns for example. But the fact that the reduction in wind speed was regional supports the interpretation that the climate system is responsible for the change.

The record does not show much decadal-scale variation. The trends in wind speed and stress show an increase from the 1940s to the 1960s, and then an almost monotonic decrease. This variation is much more in-keeping with the form of the Atlantic Multi-decadal Oscillation (AMO) that has been described by Schlesinger and Ramankutty (1994). The AMO is manifest as variation in the annual average sea surface temperature in the North Atlantic (30–65°N). Trenberth and Shea (2006) showed that water temperature (positive AMO) led to reduced storm activity and it has been linked empirically, and through models, to several regional ocean and atmosphere trends. For example, above average summer air temperatures in the eastern United States during positive AMO was suggested by Enfield et al. (2001), and Goldenberg et al. (2001) found a positive AMO anomaly was associated with increasing frequency of land-falling hurricanes on the eastern seaboard.

The December AMO index from the NOAA ESRL analysis (black line) with the 95% bounds of the FMA along Sound stress distributions. The blue shows the lower bound and the red line is minus the upper bound. All three records have been standardized through division by the standard deviation.

This figure shows the AMO index developed by the NOAA earth Systems Research Laboratory (http://www.esrl.noaa.gov/psd/data/timeseries/AMO/) using satellite sea surface temperature maps. To simplify the graphics, we divide the index by the record standard deviation. The blue lines in (a) of the figure above show the upper and lower value of the along Sound stress component that are greater than 95% of the observations in a particular year. We repeat these lines in the figure to the left after dividing by the standard deviation and changing the sign of the upper (positive) bound. The similarity in the in the pattern of the curves is remarkable. Since the wind records only extend from 1947, less than a single oscillation has been captured. This correspondence can only be further investigated through models

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A factor of two change in the stress should be expected to have a significant effect on the ecosystems of the Sound. Reexamination of data with the recognition of these long term changes have been occurring may bring new insights to the understanding of the variability in LIS.

References

Enfield, D.B.; Mestas-Nuñez, A.M.; and Trimble, P.J. (2001). The Atlantic Multidecadal Oscillation and its relation to rainfall and river flows in the continental U.S.: Geophysical Research Letters, v. 28, p. 2077–2080.

Goldenberg, S.B., Landsea, C.W., Mestas-Nunez, A.M., and Gray, W.M. (2001). The recent increase in Atlantic hurricane activity—causes and implications: Science, v. 293, p. 474–479

O’Donnell, J., H.G. Dam, W.F. Bohlen, W. Fitzgerald, P.S. Gay, A.E. Houk, D.C. Cohen, and M.M. Howard-Strobel (2008). Intermittent Ventilation in the Hypoxic Zone of Western Long Island Sound During the Summer of 2004. J. Geophys. Res., 113, doi:10.1029/2007JC004716.

O'Donnell, J. (2010). A Change in the Wind: Long Term Trends in the Forcing of Long Island Sound. Proceeding of the 2010 Long Island Sound Research Conference. http://lisfoundation.org/downloads/lisrc_proceedings2010.pdf

O'Donnell, J., R.E. Wilson, K. Lwiza, M. Whitney, W.F. Bohlen, D. Codiga, T. Fake, D. Fribance, M. Bowman, and J. Varekamp (2014). The Physical Oceanography of Long Island Sound. In Long Island Sound: Prospects for the Urban Sea. Latimer, J.S., Tedesco, M., Swanson, R.L., Yarish, C., Stacey, P., Garza, C. (Eds.), ISBN-13: 978-1461461258

Schlesinger, M.E. and N. Ramankutty (1994): An oscillation in the global climate system of period 65-70 years. Nature, 367, Issue 6465, pp. 723-726, DOI: 10.1038/367723a0

Trenberth, K.E. and D.J. Shea (2006): Atlantic hurricanes and natural variability in 2005. Geophysical Research Letters 33, L12704, doi:10.1029/2006GL026894 

Wilson, R.E., R.L. Swanson and H.A. Crowley (2008). Perspectives on long-term variations in hypoxic conditions in Western Long Island Sound. Journal of Geophysical Research, 113,   C12011, doi:10.1029/2007JC004693.

Wilson, R.E., S.D. Bratton, J. Wang and B.A. Colle (2015). Evidence for Directional Wind Response in Controlling Inter-annual Variations in Duration and Areal Extent of Summertime Hypoxia in Western Long Island Sound. Estuaries and Coasts (2015) 38:1735–1743 DOI 10.1007/s12237-014-9914-2

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